THREE-PHASE HV VACUUM CONTACTORS & CIRCUIT BREAKERS

Detailed Information

Personnel ProtectionRoss Engineering's HV control devices help users, and manufacturers meet OSHA and other safety regulations for high voltage and line to ground current protection. In as little time as 2 to 16 milliseconds (1/8 to 1 cycle at 60Hz), the integral high speed trip mechanism, driven by a Ross relay driver, will open the Ross breaker and interrupt the load current, thus minimizing, damage to the controlled equipment. When proper ground fault or leakage current sensing is used, this can be fast enough to possibly reduce injury and may possibly protect personnel from accidental electrocution as a result of contact with one line and ground, or voltages developed in the ground from ground current.

Long Life with No Maintenance These are proven, well-known characteristics of HV vacuum interrupter devices that are standard on all Ross upright and low profile HV vacuum contactors and circuit breakers. Vacuum interrupters are used because of their extremely high dielectric strength, and rapid recovery after arc extinction. This high dielectric strength exhibits comparatively low arc energy during high Current interruption, and thus minimizes contact erosion. The vacuum medium provides a very rapid but controlled arc extinction due to high velocity radial diffusion of vaporized special metal alloy contact surfaces during contact separation, thus allowing a very rapid but controlled recovery of dielectric strength. Under adverse conditions such as altitude, extreme temperatures or humidity, the interruption capability is not affected. Also, of major importance, the high-speed interruption reduces possible fault damage to equipment.

In addition to the extended life characteristics of the vacuum interrupters, the Ross HV vacuum contactors and circuit breakers are built with moisture resistant G-10 Epoxy glass as standard insulation.

Compact-Lightweight & Quiet Simplicity of design and careful use of material makes many of the Ross Engineering three-phase HV vacuum contactors, and circuit breakers the most compact, lightweight devices in their class. Both the upright and low profile models are built especially for indoor open style equipment and metal enclosed switchgear or power supplies of the smallest configurations. Despite extra strong operating forces to insure positive operation with no contact welding, the Ross low profile models are less than one-third the height, weight and size of most switchgear in the same power interruption range. And because of the reliable actuator design and the use of vacuum-sealed HV contacts, both types operate quietly and efficiently whether in air, oil or insulating atmospheres.

Multiplicity of Use The Ross Engineering Corp. HV vacuum contactors and circuit breakers, in both the upright and low profile models, are built of precision made, high vacuum interrupters and electromagnetic, or other type, actuators linked together by an insulating rod and high voltage standoff. These features, along with a flexible current transfer design that insures positive current transfer, makes these HV devices ideal for use in one, two or three-phase applications for 1/2 to 2 cycle current interruption, shorting, or transferring to emergency standby or alternate lines. The Ross three-phase vacuum contactors are suitable for AC high power and industrial controls in ranges of 208V to 38kV and to 300kV in special applications. The lightweight, low profile models are designed especially for fixed or mobile applications including use in transportation vehicles. They can be mounted in any attitude in equipmentor metal-clad enclosures, in oil, air or insulating gas. All units have excellent performance characteristics for high speed ground fault tripping and for use in mines, in portable equipment control, or in long life, minimum maintenance load control. In addition, the latching units can be fitted with a loss of control device to automatically open with control power failure.

APPLICATION DATA

Operate Voltages for HV Interrupters Ross Engineering's normally open, normally closed, latching, and trip free type HV vacuum contactors and circuit breakers operate at 208V to 38kV or more. In most standard applications, voltage ratings can be raised in multiples of 30kV (line to line) above 15kV by placing contacts in series. For example, two 15kV rated contacts in series per phase are suitable for 38kV; three to four contacts in series per phase are suitable for 75kV applications. Capacitive grading may be required.

Continuous Ratings Continuous current ratings are 200, 400, 600, 900, and 1,200 amps RMS, with up to 20,000 amps continuous with the use of a shunt switch to carry the continuous current while using the interrupt ability for the actual interruption. This switchgear has 6OHz interrupt ratings of 2,000 to 28,000 amps, 50,000 to 830,000 kVA at 15.5kV. Some 50Hz maximum interrupt ratings may be derated. Also 5,000 to 20,000 amp RMS, 10 cycle momentary, 40,000 amp RMS, 1 cycle momentary, and 100,000 amp 20microsec AC capacitor discharge.

AC & DC Interruption Contacts are designed with copper alloy combinations to limit chopping current to less than a 1 to 3-amp level on AC circuits to minimize switching transients. The copper alloy type of vacuum contact has no appreciable DC interrupt capability by itself, but it can interrupt high DC currents by use of a properly timed counter-pulse system which can simulate Current zeroes. (See other brochures for DC interrupting contactors). Transient suppression, a 0.25 to 0.5 MFD capacitor in series with a 2 to 5 ohms per kV of resistor should be used in parallel with some types of inductive loads. Self-resonance possibilities must be considered.

For higher DC current interruption, i.e. 10,000 amps or more at 20kV DC per contact, or 40kV DC for 2 contacts in series, Ross AC rated contactors with copper alloy vacuum contacts can he satisfactory. This is possible if a multiple current zero creating counter-pulse is applied with a carefully controlled rate of recovery voltage. This recovery should approximate 60Hz to 400Hz current zero and recovery voltage characteristics by means of a resistor-capacitor and switching network or In combination with an inductive ringing circuit.

Transformer Switching Transformer switching can create over-voltages in the transformer during both interrupting and closing depending on prestrike reignitions on closing and high frequency virtual Current zeros, current chopping, restrike or excessive reignitions oil opening.

Dry type transformers or inductors are particularly vulnerable since the lack of oil fully impregnating between windings reduces the capacitance between those windings and also reduces high frequency losses during recovery voltage, and allows faster recovery voltage rise time resonances and overshoot. Winding resonances from below 100kHz to over 1MHz can respond to fast rise times of recovery voltage or current and create over-voltage.

Interrupters that have multiple reignitions on closing prestrike or bounce, or reignitions, restrikes, or excessive current chopping during interruption can create severe repetitive over-voltages within the transformer windings. The application of approximately 1/4 to 1/2 microfarad capacitors in series with a low ohmic resistor, such as approximately 2 to 5 ohms per kV, connected as close to the load terminals as possible is very effective at reducing the over-voltages occurring during closing or opening. Capacitors can be full voltage rated Delta phase to phase, or WYE grounded or ungrounded to reduce the capacitor voltage rating requirement. Station type MOV type arresters have also been helpful but are not as effective since they do not reduce the fast rise times, although they help to reduce peaks.

Inductive Load Switching With highly inductive loads, wherever possible, station class transient suppressing non-linear resistance and lightning protective capacitors in series with 2 to 5 ohms should be placed across the load as close to the load equipment terminals as possible. Older lightning arresters with Internal spark gaps or even MOV types have not been found as effective as closely connected protective capacitors in series with low ohmic value resistors for this duty. This is particularly essential for repeatedly switched inductive loads such as dry type transformers, arc furnaces, motors and low current, transient generating inductive loads.

Caution: capacitor values must be selected to prevent self resonance.

Most switching devices can create over-voltages on switching, Experience has shown that for iron core oil filled reactive loads, normal switching over-voltages of 2 to 2-1/2 times operating voltage are to be expected with almost any type of interrupter. Air or gas insulated and particularly air core inductive loads can generate even higher over-voltages if there is insufficient shunt partially damped capacitance.

Ontransformers and other iron core inductive loads, normal inrush currents of 5 to 10 times rated load current are expected, depending on the degree of magnetic retentivity from the previous interruption. If iron core inductive loads can be re-energized on the opposite polarity from which they were de-energized, then inrush is minimized, otherwise it is limited primarily by the winding resistance only Repeated high inrush closing causes great mechanical stress on the transformer windings, as well as rapid erosion of the vacuum contact (which can be over 10 times greater on closing than on interrupting even the same current). Therefore, transformers that have marginal insulation and bracing can deteriorate from mechanical movement with repeated switching. Probably a number of transformer failures occur because of these high inrush Currents and poor bracing, not from over-voltages.

Closing & Inrush Currents Where the high inrush and repeated switching problem is serious, Ross Engineering recommends a step start arrangement with a dual vacuum contact system. In this instance, one contact unit isequipped with an inrush limiting resistor of about 30% to 100% of the value of the full load impedance, which is inserted initially for a few cycles before the main vacuum contact is closed.

Actuator Types & Voltages These units are available with line or energy storage close and trip with either trip and anti-pump or automatic reset actuators. 230V or 480V, 60Hz is the standard non stored energy actuation voltage; 115V, 209V, 240V and 480V, AC, 125V DC, and 290V DC, 700 microfarad capacitor trip are also common when applicable. Many other actuator voltages can also be accommodated. When ordering, actuator should be specified as well as HV operating voltage and current, basic impulse level, maximum interrupt, continuous RMS current, 1 cycle (17 milliseconds) fuse protected or 10 cycle (167 milliseconds) momentary RMS currents, number of auxiliary contacts required, type of load, number of operations per year, speed of opening and closing and any other specific requirements should be stated.

Ross High Speed Transfer Breaker in Action

To help solve high speed alternate source transfer problems, Lockheed's Satellite Test Center in Sunnyvale, California, installed two Ross Engineering Corp. mechanically interlocked 12kV three-phase power transfer circuit breakers. The Test Center's huge computers control missile launches from Southern California's Vandenberg Air Force Base. During power line failures, the Ross breakers automatically separated the entire center and transferred it to another 12kV line within milliseconds, thus avoiding major computer and launch recycling.

Data Generally, the high speed EV power transfer breakers are used for AC loads 1,000V, or less, to 15kV, 600 amps continuous or less; and for instantaneous or delayed transfer to alternate or standby power source, or for high speed system separation. Power outage time can be held to as low as I cycle (17 milliseconds) or can be ½ cycle to 1½ cycles trip + interrupt, or make before break on return where suitable. Optional delay system will give adjustable delays of 2 cycles to 30 seconds or more before transfer to prevent unnecessary transfers, if desired. Some units can be mechanically, as well as electrically, interlocked to prevent possibility of two alternate power sources being tied together at any time. Contact Ross Engineering for further data.